Shadows move faster than light?

[robespierre]

Uhhh... I think I know where you're going with this..... Are you trying to say that darkness, absence of light, whatever moves faster than light? I wish I had math to back it up, but I'm too lazy, so how about the empirical method....? I have noticed in the past that a shadow is not the exact same size as the object creating it. Depending on a number of factors it will either be larger or smaller, but rarely the same size (Usually larger/longer)... So perhaps the reason you are experiencing this is that the larger (even if only slightly) shadow is taking up more space, giving the illusion of added speed. If a person with long legs and a person with short legs, were both to walk at the same speed, would not the long legged individual always seem to be ahead, even though they are both travelling at the same speed? That's my thought about it anyway... But at any rate, How could darness travel faster than light, when it is merely light that has been prevented from reaching it's destination?

------------------
I can solve any problem!
Just give me a rifle and a clocktower!

[metroshane]

Back the light away from the 'light blocker' and see if you come up with comparable data.

I think it has more to do with geometry than physics.

------------------
I treated Art as the supreme reality, and life as a mere mode of fiction--Oscar Wilde

[C K Dexter Haven]

Who knows how the shadow can travel faster than light? Nyahahahaha.... the Shadow knows.

It has to do with a secret learned in the Orient, how to cloud men's minds.

[Andy]

I wondered about this problem myself, but can't remember if I posted it anywhere. Anyway, it's obvious that nothing physically moves faster than c, but can the lightness/darkness at the two extremes be considered to be 1's and 0's ie has transmission of information occurred faster than c? Is this forbidden?

[Triskadecamus]

Ok, engage imagination, and visual aids.

Take a penny, and a quarter. Put them on a table. The quarter is the Sun; the penny is the Earth. We will ignore the geometry of Penumbral and Umbral shadows, and consider the shadow to be infinite. You are the observer, six hundred million miles from the Earth. You watch as the Earth passes in front of the Sun, and see a shadow. The image of the Earth is twenty seconds old, when you see it. The position of the Earth is really a bit foreword from the place where it was when it cast the shadow you see when you see the eclipse. The shadow is curved, into a spiral, reaching off into the distance. Move the earth faster, and the pitch of the spiral increases, but no part of the shadow moves faster than the speed of light.

Geometry is so beautiful.

<p align="center">Tris</p>

[pluto]

Another way to see that the shadow doesn't move faster than the speed of light is to consider the outgoing light beam as a stream of photons instead of a rigid beam. When you rotate the source you don't affect the photons already en route. The new photons don't alter the shadow until they have travelled the length of the beam at light speed. The light would follow the spiral our thirteen-dimensional friend described.


------------------
"Vandelay!! Say Vandelay!!"

[Andy]

Maybe I should have read the OP a bit more carefully.
My scenario is this. Holding a torch of some sort, I direct the light at some point "A" on a wall and move it through an arc of arbitrary angle to point "B". As I walk further from the wall, repeating the same action, the apparent speed at which the shadow 'moves' increases. I cannot see a limit to this increase in speed, and although I appreciate that no physical object is moving at any spped greater than initially, haven't I sent a signal (information) (ultimately) from point "A" to point "B" at speeds faster than light ? Is this allowed an allowed process?

[Keeves]

Okay, lemme try this again, I obviously did not explain myself well enough. I got this idea from a discussion elsewhere about rotating a disc so fast that the outer edge exceeds lightspeed. Someone responded that the atoms would break apart at close-to-light speeds, and the experiment would fail. So I came up with this idea which is independent of any object which might break. But I think it will be easier if I talk about light beams instead of shadows.

The radius of Earth's orbit around the sun is about 150,000,00 km. That makes a circumference of about 950,000,000 km. I am near the sun, and I have a powerful laser with me, shining a powerful light. I rotate this laser at the very lazy speed of one revolution per minute. It will shine on one part of the earth, and then another part of the earth, and another, and then it will shine into various parts of empty space until it completes one revolution along earth's orbit.

My point is that the shiny dot which it is placing on the earth's surface will move across that surface at the speed of 950,000,000 km (the circumference of earth's orbit) per minute (the laser is spinning at 1 rpm, remember?). This is equivalent to 16,000,000 km per second, which is FIFTY TIMES faster than the speed of light (300,000 km/sec).

Example: The moon is about 385,000 km from the earth. Therefore, it will take about 1/40 of a second for *this* spot of light to get from the earth to the moon, but it takes about 1.3 seconds for light from the earth to go to the moon directly.

No physical laws are broken here. All the photons leave our laser at the same 300,000 kps. The only thing is that it took 1/40 of a second for the laser to rotate from facing the earth, until it was facing the moon. A shadow would do the same thing, if an object would move past the sun at similar speeds.

But I cannot think of any practical usefulness of this idea. Can anyone else?

[RickG]

In the previous example, (where you shine a flashlight on the wall and the spot moves faster than light from point A to point B, no information has actually traveled between the two points. All the information transfer is from the flashlight to the wall, and that is traveling exactly at c. Relativity only claims that information cannot travel FTL. The ultimate moving flashlight is a pulsar. It sends out a beam of EM radiation while rotating at high speed (IIRC, millisecond period pulsars are roughly Earth-sized--imagine a thousand sidereal days passing per second!). Imagine a millisecond pulsar 10 light-years from Earth. The circle the beam traces in space with radius of that distance is 62.8 ly in circumference (2Pi * r), but it traces the circle in 1/1000 sec. So the speed of the spot is ~63000 ly/sec! This is no problem for relativity though. Imagine a big mirror on the other side of the circle (i.e. 20 ly from Earth) reflecting the beam spot back toward us (with enough clearance to miss the pulsar in the middle on the way). It still takes 20 years for us to see that the beam hit that spot, even if the photons that hit us and the mirror were emitted 1/2000 sec apart. So, no FTL information transfer. Sorry.

Rick

Rick

[Keeves]

android209 posted his while I was writing mine. Yeah, you got my point. My problem is that since the light originates from the sun (or the torch) rather than going from the earth to the moon (or from A to B), I don't think there's a way for any information to get from hee to there faster than light.

But maybe someone can build on this and come up with something.

[Cooper]

The problem is that it is not moving faster. It is travelling the same distance in the same amount of time. If you don't believe me, then actually time it instead of just looking at it. It may appear to move faster but it is not.

[RickG]

Keeves,

My example was even more extreme than yours, but essentially the same. Does the mirror on the other side help resolve the apparent paradox?

Rick

[Keeves]

Thanks, Rick, but I think we're stuck. No matter how we set it up, the shorter information path is the one to deliver the information first.

Pulsar to earth is shorter than pulsar to mirror to earth. Sun to moon is shorter than sun to earth to moon. Torch to A is shorter than torch to A to B. The difference between the two routes is faster than going directly from one destination to the other destination, but that doesn't seem to help. We don't want it to be there faster, we want it to be there beforehand, and that's not happening.

[RickG]

Keeves,

I think maybe we're saying the same thing: There is no paradox, because you can't transmit any information FTL this way, even though the shiny spot does move FTL (that is, the two events 'light spot hits point A' and 'light spot hits point B' have a space-like separation).

Best,
Rick

[MrKnowItAll]

I'm going out on a bit of a limb (and I'm sure the flame-throwers will be following me) but I'm going to take a stab at this.

If you move the bright point of a laser from point A to point B. There is nothing real that is being moved. You are only changing the direction of the photons you are shooting. The bright spot where it hits isn't an actual thing and cannot be used to move faster than light. To use it to send info, you would have to send the info to it's source first, and then the source could relay it to another location. Still pretty quick, but certainly not FOL.

The laser, BTW, is just an easier thing to describe than the shadow, but on the whole it still works the same way. However, I disagree with the earlier posts. I may be wrong on this, but I think a shadow can move FOL. After all, it has no mass and no inherent energy. It is merely a void, an absense of light. Practical uses? None that I can think of.

------------------
To do is to be. -Descartes
To be is to do. -Voltaire
Do be do be do. -Sinatra

[tracer]

Light doesn't move, darkness moves.

http://pweb.netcom.com/~rogermw/darksucker.html

[MrKnowItAll]

LOL, tracer! You should post that over in the "Weird Websites" thread in MPSIMS, if you haven't already.

[Keeves]

The following idea once occurred to me:

Take a light source, and a flat wall of some kind a certain distance from it. Now put an object of some kind halfway between the two. If that object moves, its shadow on the wall will move twice as fast as the object itself. If the object moves at 3/4 of the speed of light, then the shadow will move across the wall at 1.5 times the speed of light.

Am I making an error somewhere in this? I'm sure that someone, somewhere has discussed it. Any practical uses for this effect? Thanks!

[SoulFrost]

Well, a shadow isn't a "thing"...it's a region of reduced light. As I see it, the measurement you need to use in this case isn't the speed of the shadow, or the speed of the photons, but the total number of photons. Fewer photons in an area = more shadow in that area.

-David

[handy]

Wait a moment, are you people forgetting the obvious or is it I?

If you put a bottle on a table, light travels around & in back of the bottle. The shadow is not a true shadow because some light goes around the bottle. Thus, some of that light takes a path sligthly longer. Thus it would be reasonable to assume that light is a little slower than what light doesn't have to go in back of the bottle.

[douglips]

Handy writes (babelfish (http://babelfish.altavista.com/) translation):
If you to put a bottle on a table, the light travels around &amp; inside it stops backwards of the bottle. The shadow is not an applicable shadow, because something light the bottle goes around. Thus, some of this light slightly makes examination of a passage more for much time. Thus it would be reasonable to assume that the light is the one little slowest one of the one than that light does not have that to go inside stops backwards of the bottle.

I think you are saying that near the shadow of the bottle the photons are slowed down by the nearness of the bottle to their path, but I'm having a hard time understanding your point. If my interpretation is nearly correct, then I still don't understand how this affects the speed of the shadow.

Assuming the mass of the bottle is less than the mass of, say, the planet Earth, the photons will be nearly undisturbed by gravity. You may get some diffraction effects near the edge of the shadow, and the photons may be deflected a very slight amount, but this effect goes away even mere millimeters from the bottle, so that light very close to the shadow is still unaffected by the shadow. Even the diffracted light will still be travelling at the same speed, just on a very very slightly longer path, such that the difference in travel time can be ignored.

How would any of this affect the speed of the shadow/information transmission/etc?

[Homer]

If two men sat chained in a cave their entire lives, seeing only the shadows on the wall, and hearing voices on from behind them, and seeing nothing but the shadows on the wall, if one man broke free and saw the what caused the shadows, when he returned to tell the other, would he believe him? Would he understand him? Could he understand him?

Who understands this?

--Tim

------------------
We are the children of the Eighties. We are not the first "lost generation" nor today's lost generation; in fact, we think we know just where we stand - or are discovering it as we speak.

[mr john]

I understand it tim. I been operating in the dark a looooong time. I am presently developing the faster than dark galactic explorer. A lot of physists are ridiculing me, even while they continue work on BLACK holes. The answer is right there in front of them yet they don't see it! Well, yeh, what with it bein' dark and all I guess it is hard to see.

------------------

"Pardon me while I have a strange interlude."-Marx

[C K Dexter Haven]

Homer -- it's been done, chap named Plato.

On the serious question posed by Keeves, I think Pluto's answer is the most cogent.

You are thinking of the shadow as moving simultaneously with the light source, but in fact the shadow lags the movement of the light source (by the delay it takes the light to travel that distance.)

In the example of standing at the sun and shooting a laser out to the earth, and you rotating at 1 rpm.. You're thinking of the laser as if it were a long straight pole, so that the "end" that hits the earth moves faster than the center (like on a rotating wheel, where a point on the circumference moves faster than a point near the center.) Think of the light stream instead as a series of little ping-pong balls being shot from your laser-gun, and I think the seeming paradox will go away.

Also remember that, in relativistic physics, you don't just add velocities: at low speeds, you can add velocities, but when speeds get up towards c, there's a factor of c^2 in the denominator that limits the speed.

[JoeyBlades]

So far, the explanantions have not been to my liking, so here's my two cents worth. For the purposes of this discussion light can be viewed in a fairly classical manner as a stream of photons. Each photon has a constant fixed (and maximum) velocity.

Shadows cannot move at faster than light speed... neither can they move at slower than light speed. The barriers that cause shadows DO move at slower than light speed and while the apparent shadow can perhaps move faster than the barrier itself, this apparent movement can never exceed the speed of light. If it were possible to approach the speed of light with this apparent movement, the photons that just barely escaped being blocked by the barrier would continue on toward the wall at light speed, and so to would the photon gap (i.e. shadow).

To help you visualize this, let's slow things down a bit. Let's say that we're in a vacuum in space. No friction or significant gravity to skew our experiment. Let's say we have a special emitter that shoots out pellets towards a wall. The emitter fires multiple, parallel streams of pellets at a constant velocity, let's say 1 meter per minute, and each pellet lags it's predecessor by 2 seconds. Now let's say you have an observer at the wall and he's watching the pellets strike the wall. Now we'll introduce a barrier in front of one of the emitter streams and let that barrier sit there long enough such that no pellets from that stream are striking the wall. No pellets = a shadow. Now start to move the barrier across the other parallel pellet streams. At any point where the barrier interferes with a pellet stream, all of the pellets that arrived at that point prior to the barrier continue on their way toward the wall at the same velocity and therefore the gaps in the pellet stream are constrained by the same velocity. To our observer, the "shadow" moves at exactly this same velocity or slower.

Hope that was helpful.

[MrKnowItAll]

Nice job, Joey. You made me think for a minute. However, I don't buy it. Here's why:

In your example everything is moving in a parallel way (if I'm understanding you correctly). Let's alter it a bit.

First, all of the pellet shooters are in one place shooting from one point. Second, the pellets shoot out in waves in 360 degrees.

Now then, the pellets radiate outwards at the meter/minute rate you suggested, but as they go they get farther apart from each other.

Of course, you are correct that pellets that are en route before the barrier arrives to block further pellets will continue on their way. This is what causes the "spiral" effect that was mentioned in the previous posts. Still, the effect of the gap caused by the barrier will grow more pronounced the farther the pellets travel. The gap will grow larger and move faster.

So, what do you guys think. Am I right, or just plain goofy.

[RickG]

MrKnowItAll,

You are quite correct. Once again: the position of the spot/shadow (measured in some direction oblique to the beam itself) may move arbitrarily fast. However, information cannot be transmitted FTL this way, so there is no violation of special relativity.

The shadow/spot is not a "thing". The time at which the spot hits a given point in space is an "event"--a point in space-time. The continuous series of "events" that trace the space-time path of the spot over some giant screen may be connected in a space-like manner (if the spot moves FTL) or in a time-like manner (the spot moves STL). Only in the latter case can one transmit information with the spot--and that information is transmitted at less than c.

Rick

[JoeyBlades]

MrKnowItAll,

Perhaps I confused you by oversimplifying, but it does not matter if the pellet streams are parallel or a radial fan. Each pellet in the stream still has constant velocity and the shadow cannot occur at some point on the wall until the last unobstructed pellet from that stream reaches the wall.

[DougC]

- - - If a black hole is capable of entrapping light, then can it be said that gravity acts faster than light? - MC

[jab1]

Theoretically, no. Someone is trying to set up an experiment to measure gravity waves to verify their velocity and other characteristics, but it's very tricky, since they have to account for the gravitational effects of everything else. Every material object generates its own gravitational field, not just planets, moons and stars: You, me, your computer, its mouse and keyboard, a ball-point pen... everything has its own gravitational field. Even air generates gravity.

Anyway, if a star explodes in a supernova, to give one example, that source of gravity is now greatly weakened. This weakening should be detectable and, according to the theory, would travel outward in all directions in a spherical shape. Theoretically, it would do so at the speed of light.

I read this years ago in an old Discover magazine. I don't know if the experiment has been conducted yet.

------------------
Fighting my own ignorance since 1957.

[jab1]

Another update:

How about a shadow 153 light-years long? A few days ago, the existence of a planet orbiting another star was proven beyond the shadow of a doubt. :) (I can hear you groaning from here.) Go to: http://cnn.com/TECH/space/9911/13/newplanet.ap/index.html to read how this was done. It's a short article. To sum it up, this planet moved between us and its star, eclipsing it. The event was detected with a telescope.

Maybe using this, y'all can figure out how a shadow can't move faster than light.

------------------
Fighting my own ignorance since 1957.

[ravenous]

Okay, say I have a flashlight that shoots a really straight beam of light. I tape a picture of Pikachu on the shiny end of it.

Now I shine the light at a big square 1 light year across and 1 ly away. I start at one side and shine Pikachu across the face of the square in 1 second. It takes just a flick of the wrist.

Bingo, Pikachu the shadow is moving at 1 ly per second.

I know this has no practical value but it's also valueless to argue that it's impossible.

Didn't you ever play with a water hose? The water moves at velocity x but the point where the water hits the side of the house (whoops) can move as fast as you can wiggle the thing * the distance or something.

Rav

[MrKnowItAll]

Okay, JB, lets try this again.

(Ahem.) We have our radially dispersing pellet gun firing the same way as described before. Now, we mark a point in space two feet from the gun and call it point A1. Two feet away from A1 and the gun we mark point B1. This forms an equilateral triangle A1B1C (assuming the gun is point C). Next we draw a line through A1C and mark point A2 one lightyear from C, making line segment CA1A2. We do this again this time through B1.

Now we have another equilateral triangle CA2B2, which is one lightyear on each side. Now we build a rather long wall between A2 and B2 for the pellets to hit.

(Now, I know that all the preceding is probably obvious, but I wanted to make sure that everything is well defined and labeled before going on.)

We start shooting pellets radially as previously described. We let this go for a long time, until the pellets start hitting the wall. Next we put something in the way of the pellets at point A1, lets say Nickrz's head. (Ain't thought experiments fun?) This will eventually cause a pellet gap at A2. Next, we move Nickrz's head in a straight line to B1, taking exactly one second and obstructing pellets as we go. Then we give Nickrz some Bactine and tell him to go away.

The gap will form a slightly curved line (due to the radial nature of the pellet distribution) that radiates outwards toward the A2B2 wall. Because the pellets are getting farther and farther apart as the progress outwards, the gap in turn gets larger and longer. Eons pass. Finally the pellet gap begins hitting (not hitting?) the wall. The gap will take one second to move from A2 to B2, moving one lightyear in one second, well over "c". This is because the shape of the curve will not change, only its size.

For your example to be correct, JB, the shape of the curve would have to change, indicating the the speed of the pellets is not constant. The speed of light, as we all know, is constant.

Is that clearer?

[mr john]

Hmmm, Pluto shows up from the dark underworld, Handy starts throwin shadow puppets on the wall, and how about Robespierre for your shady characters? And ,Dex, you gotta cut Homer some slack, not only was he dead before Plato was born, he is blind too,never seen a shadow. But yall have lost me. If I have achieved dimness it is because I have stood in the shadow of giants.

------------------

"Pardon me while I have a strange interlude."-Marx

[jab1]

ravenous, what you have done is create an equilateral triangle: each of its three sides is exactly one light-year long.

Turn on your flashlight. It takes one year for the (let's assume) circular pattern of light (and Pikachu's shadow) to reach the cube face. Ten seconds later, flick your wrist to the right and you're now aiming at the opposing side of the cube face. Wait one year. Now Pikachu's shadow is hitting that other side one year and ten seconds after it hit the other side. How long did it take for Pikachu's shadow to cross the distance between the two points?

One year. Why? It took that long for the light to get from the flashlight to the cube. The pattern of the photons would no longer be a nice, coherent, easily-seen circle, but a cylinder stretched out over the entire distance of the cube face because each wave of photons would be hitting to the right of the wave before. After your hand stops and one year passes, a circle is again visible.

In other words, since your triangle is one light-year on each side, it will take light one year to travel each side, no matter where that source of light is.

------------------
Fighting my own ignorance since 1957.

[jab1]

I don't know why I wrote cube instead of square. Oh, well....

------------------
Fighting my own ignorance since 1957.

[douglips]

jab, that is just plain wrong.

1. Aim the flashlight at point A. Wait one year and ten seconds.
Note that Pikachu has been shining on point A for ten seconds.
2. Flip your wrist to aim the light beam at point B gradually over a period of 1 second.
Pikachu continues to shine on point A for one more year, because you aimed at point A for one year and 10 seconds.

After one year and 10 seconds of shining on point A, Pikachu moves within 1 second to point B, no matter how far away point A is from point B.

Your example seems to say that despite aiming the flashlight at point A for one year and ten seconds that pikachu is only visible at point A for ten seconds and then departs at velocity = c for point B. How did it know when to leave to go to point B? If you are a light-year away, it will take a year for pikachu's image to start moving.

See the above garden hose analogy, and go water some plants and you'll see exactly the principle at hand.

In other words, since your triangle is one light-year on each side, it will take light one year to travel each side, no matter where that source of light is.
But nothing is traveling from point A to point B. Things are travelling from point ME to point A, then very quickly things start traveling from point ME to point B. When photons I sent to point B get close to the wall, the photons in between point A and point B start hitting the wall. Where they hit can move arbitrarily fast.

[Lord Jim]

I would like to add to this, MrKnowitall's fan shaped experiment seemed like the right idea, but A1B1 and A2B2 should be arcs instead of straight lines. If modified such that there are 12 streams of pellets/rays of light , whatever, Nick's head has to actually move into stream 1 across the seperation between stream 1 and then into stream 2 and so on. When he's all the way across, he is much more concerned with who pushed him than what the results are. But watching the results a couple of years later, we see a dark area seemingly moving across the 12 streams. Me and The Shadow know that it did not move from the point of impact of stream one, sneaking in the darkness across the seperation between stream one and 2 and then into stream 2. Actually, stream 1 turned off, some time later stream 2 turned off. No movement at all. If Nick got pushed through the streams in 1 second, what we see later is 12 stream over a light year across being turned off and back on in 1 second. What we interpret is that something moved across that distance in a second. It didn't happen. We could create the same visual effect by dropping Nick and 11 of his friends successively through the streams over a period of a second. (I don't know why he keeps volunteering for this.) We know all the movement was vertical, and I'm sure Nick does too. But, later our perception at the wall is the same, a dark spot moved across the entire width of the wall in 1 second.
There is no rapid movement exceding the speed of light or anything. The spots on the wall are successive shadows, not "A" shadow moving. The shadow that follows me around is not the same one that follow everyone else, (although, they do talk to each other, I've seen them use this keyboard, actually).

[jab1]

Let's see...

Shine flashlight for ten seconds at target one light-year away. Then flick wrist to right to shine light at different point. The light then would form a continuously spreading fan pattern going out from your point and heading toward the target at the speed of light. (The pattern would spread forever if it never hits our target.)

One year passes. The first photons hit the edge of the square. Ten seconds later, the fan pattern would then hit the square and would form a pattern one light-year long that takes one second to form. (That's how long it took you to flick your wrist to the right.) Then the familiar image of Pikachu appears at the opposite edge of the square.

Is that it?

------------------
Fighting my own ignorance since 1957.

[MrKnowItAll]

Yep.

[JoeyBlades]

jab1 wrote:


Someone is trying to set up an experiment to measure gravity waves to verify their velocity and other characteristics...


Einstein already calculated the speed of gravity and found it to be exactly the same as the speed of light. There may be experiments, as you describe, but they can only be used to further verify Einstein's theory of general relativity.

In answer to MC's question: A black hole captures a photon when that photon gets closer than the event horizon of the black hole... actually, that pretty much sums up the definition of the event horizon. It's not the velocity of a photon versus the velocity of a graviton, that's at issue. It's that the force of a gravitational field gets increasingly stronger as you approach it's mass. When a photon gets close enough the force of gravity will prevent it's escape.


To sum it up, this planet moved between us and its star, eclipsing it. The event was detected with a telescope.

Maybe using this, y'all can figure out how a shadow can't move faster than light.


Sorry, jab1. All that means is that the shadows that we see today are the result of 153 year old movements. If the planet were to disintegrate today, we would continue to witness the orbit of this planet every 3.523 days for the next 153 years...

[JoeyBlades]

ravenous

You wrote:


Bingo, Pikachu the shadow is moving at 1 ly per second.


You've changed a few of the parameters with this example. Now you're panning the light source, not obstructing a steady source. But just for grins, let's look at the implications of what your change means. Let's say you've got a machine gun with lots of ammo. As you fire your machine gun at the wall and pan very slowly from left to right, you will cut a line in the wall. However if you pan the machine gun very fast, the line will disperse into a sequence of holes. The same thing will happen with your sweeping light source. The photons don't get there any faster, however if the rate of change in distance from where one photon strikes the wall and the next exceeds the rate at which photons are arriving, there will be jumps between photon strikes, but these are not the shadow that was transmitted from your masked light source, but rather the darkness that was always there.

[JoeyBlades]

MrKnowItAll,

I see where you're coming from and agree that the rate of the shadow across the wall will not be linear, however consider this. As each pellet approaches the wall, the perpendicular component of it's velocity with respect to the wall will be less than it's velocity perpendicular to it's point of origin, except for the stream of pellets that are perpendicular to both the source and the wall. So in other words, the velocity of the shadow, if you will, will necessarily be less than or equal to the velocity of the pellets. It can't be faster.

I previously wrote:


... neither can they [shadows] move at slower than light speed.


I now concede that this part was bunk; I was considering only shadows that are cast within a parallel stream of photons. All I can say is "Oops!" The first part is still correct:


Shadows cannot move at faster than light speed...

[ravenous]

Joey, I have the following problems with your post.


As each pellet approaches the wall, the perpendicular component of it's velocity with respect to the wall will be less than it's velocity perpendicular to it's point of origin, except for the stream of pellets that are perpendicular to both the source and the wall.


What does it mean to be perpendicular to a point? I'll be generous and say you mean perpendicular to a line parallel to the wall going through the point. The component of the velocity in either parallel line, of course, will be identical, so maybe I shouldn't be generous.

No one is saying that the pellets will move faster or slower. They will move in a straight line. But another pellet will come later and it'll be X meters farther along the wall in less than X/c seconds.


So in other words, the velocity of the shadow, if you will, will necessarily be less than or equal to the velocity of the pellets. It can't be faster.


But the velocity of the shadow and/or the point where the pellets hit the wall is independent of the velocity of the pellets!

How about an even simpler example? I shoot two pellet streams at a 1 ly square 1 ly away from me. They hit the wall simultaneously in 1 year.

Now 1 second after I start shooting I cover one of them for 1 second followed by the other one for 1 second.

Bam! 1 year and 1 second later there's a shadow at one end of my imaginary square and 1 year and 2 seconds later the shadow has moved to the other side of the square.

Rav

[ravenous]

JB said:

these are not the shadow that was transmitted from your masked light source, but rather the darkness that was always there.


Ah ha! But Ravenous' theory of shadow relativity says that the two are one in the same.

[MrKnowItAll]

Okay, JB. Let me see if I'm understanding you correctly. Let's say, using my previous experiment (the one involving Nick's head), that we add an additional element: We station another single-shot pellet gun at point A2 that shoots at the same speed of 1 meter/minute. Furthermore, we aim the gun at point B2.

Now, when the gap from Nick's head starts moving away from A2, we fire the pellet gun. Are you saying that the fired pellet will keep up with the gap as it moves to B2? That although it only took a second to move Nick's head from A1 to B1, it will still take 11,943,874 years for the gap to move from A2 to B2? (I finally got around to working the numbers.)

If that's the case, read ravenous' previous posts. I know that we all have had it pounded into our heads since 1st grade science class that nothing can exceed the speed of light. Keep in mind that "nothing" is exactly what a shadow is. Shadows are not real objects, and do not behave like real objects. They are a state of absense. As I said before, they have no mass and no energy.

In fact, the movement of a shadow is really more of an optical illusion. One microsecond, these photons are being blocked. The next microsecond a whole new set of photons are being blocked. Nothing has really moved (except for the photons and whatever is blocking them).

[shiningnight]

If you walk outside and look up at the stars facing east then turn your head and look west taking 2 seconds
( wear a mining helmet if you want to do the flashlight thing ) lets say your gaze started at point A a star 100 ly to the east of us then went to B a star 1000 ly to the west. It only took 2 seconds for your gaze to cover
the entire sky not just the 1100 light years between A & B. Because nothing actually moved between A & B
100 years later an alien looking thru his telecope after his sun (A) vanishes behind his planet for the evening and says Ha, look at that geak in a mining hat. Nine hundred years and 2 seconds (not 1100 or 1000) another alien makes the same observation but this time from around star B. Light from your geaky mining helmet headed toward A then 2 seconds later toward B. A & B are 1100 ly apart but what matters is the distance from the sorce. Alien 1 seen the light just 900 years before alien B. even though they're 1100 light yrs apart. If we'd looked west first alien B would still not see it until 900 years after A. Light isn't clearing 1100 ly in 900 yrs but thats the time in between event A (light from helmet hitting star A ) and event B (light from helmet hitting star B )
I don't see why a shadow or light or
anything else couldn't move across one point then another faster than light could move between the two.
They are independent events.

[JoeyBlades]

OK ravenous, I'll concede that the shadow of Pikachu is everywhere and that we merely fail to notice it due to a lack of surrounding photons necessary to highlight it...


What does it mean to be perpendicular to a point?


I meant perpendicular to the plane that is the wall.


The component of the velocity in either parallel line, of course, will be identical, so maybe I shouldn't be generous.


I'm not sure what you're trying to say here, but it doesn't sound right. Pellets that are on the outer streams of our 'spray' will take longer to reach the wall than pellets in streams that are closer to the center of the 'spray'. Therefore their velocity component that is perpendicular to the wall must be smaller than the pellets that are aimed straight at the wall. I don't think I can state this any more plainly...


Bam! 1 year and 1 second later there's a shadow at one end of my imaginary square and 1 year and 2 seconds later the shadow has moved to the other side of the square.


Ahh... Now I see your point. So using this same approach, let's say I have a laser aimed at the left side of the wall and another laser aimed at the right side of the wall. Initially, the first laser is turned on and the second one is off. I then simultaneously turn off laser one and turn on laser two. By your theory I have just done two amazing things (1) I made a point of light move faster than light from the left side of the wall to the right and (2) I've made a shadow move faster than light from the right side to the left side.


Alright, you've convinced me... Einstein was a putz [sarcasm].

Actually, I see the difference between both your's and MrKnowItAll's viewpoints and my own. You guys seem to define a shadow as merely the absence of light, where I require that a shadow must be the absence of light where there once was light. At any point along the wall (and I mean point to be the diameter of a photon - whatever that is) where a shadow is approaching, first there will be a photon, then there will be the abscence of a photon. This abscence cannot arrive at the wall any faster than the photon that preceeded it. Likewise, the absence of a photon at the next point along the wall cannot arrive any faster than the photon that preceede it. The shadow is necessarily gated by the speed of the photons.

[douglips]

JoeyBlades writes:Actually, I see the difference between both your's and MrKnowItAll's viewpoints and my own. You guys seem to define a shadow as merely the absence of light, where I require that a shadow must be the absence of light where there once was light. At any point along the wall (and I mean point to be the diameter of a photon - whatever that is) where a shadow is approaching, first there will be a photon, then there will be the abscence of a photon. This abscence cannot arrive at the wall any faster than the photon that preceeded it. Likewise, the absence of a photon at the next point along the wall cannot arrive any faster than the photon that preceede it. The shadow is necessarily gated by the speed of the photons.
I think the real difference between your definitions of shadow is that MrKnowItAll and I are calling the image on the wall at the destination the shadow while you are calling the hole in the photon stream as it rushes from source to destination the shadow. Let's call my definition of shadow the dougshadow, and yours the jbshadow. Of course, the photons are moving at the speed of light and of course the jbshadow is also moving at the speed of light.

However, the dougshadow which is the locus of points described by the intersection of the jbshadow and the surface of the wall can move arbitrarily fast.

In the one light-year example, it takes one year for any of the changes you make at the source to make it to the destination. But, once those changes reach the destination, the changes take the same amount of time as it took to make the changes at the source. Your two laser example above is a perfect example of this and says nothing about Einstein being a putz. The jbshadow caused by turning off one laser moves at c, and the newly turned on laser beam moves at c. The equivalent of the dougshadow which is a geometric construct and not anything physical, does in fact transit the face of the square instantaneously.

Change your experiment slightly so that you go back to the laser being swept from one side to the other in 1 second and you'll see. It takes a year for the changes to get there (jbshadow) but once the changes get tot he wall, it (dougshadow) moves one light-year in one second.

Finally, the movement of a shadow is exactly equivalent to the movement of a lightbeam - you are just inverting photons to an absence of photons, but the answer is identical except in darkness instead of light.

[MrKnowItAll]

Aha! I see now. JB is talking about the shadow en route to the target. Yes, I concede that that shadow cannot exceed the speed of light (or pellets or whatever).

[jab1]

That's the reason I mentioned that planet eclipsing its sun. The shadow of that planet cannot arrive here any sooner than the light surrounding it.

Joey, I do know what it means if a star is 153 light-years away. It means the shadow of that planet and the light of the star both got here 153 years after they were made.

Technically speaking, though, there really is no such thing as a shadow. It's an absence of light. How can an absence of something be something? Our eyes detect photons and our brain interprets the image. Our brain interprets the lack of light as a thing even though there is no thing there. (I deliberately wrote "no thing.") It's an illusion. Mechanical devices like cameras also detect photons, but, again, our brain interprets the dark areas of the subsequent images as a thing we've come to call a "shadow".

------------------
Fighting my own ignorance since 1957.